The present invention relates to an improvement of the mechanical properties of a natural rubber latex film that has been prevulcanized, particularly with radiation. More specifically, the present invention relates to a method of improving the tear strength of a radiation vulcanized natural rubber latex shaped article by shaping a mixture of a radiation vulcanized natural rubber and a water-soluble polymer. The invention also relates to the thus improved rubber latex shaped article.
Radiation vulcanization of natural rubber latices does not involve the use of sulfur or dithiocarbamates and, hence, shaped articles of radiation vulcanized natural rubber latices do not contain nitrosoamine and have an advantages of low cytotoxicity. In addition, they give rise to small amounts of SO.sub.2 and ash after burning.
On the other hand, the radiation vulcanized natural rubber latex shaped article solely relies upon the interlinking of rubber molecules for the binding of rubber particles and, hence, suffers from the problem of low tear strength.
The present invention solves these problems by providing a latex product having high safety in various biological aspects. With a view to improving the tear strength of latex shaped articles, the present inventors previously proposed a method of adding an emulsion of poly(methyl methacrylate) to a radiation vulcanized natural rubber latex. However in that method, about 30 parts by weight of poly(methyl methacrylate) had to be added to 100 parts by weight of the rubber and this resulted in the loss of the advantages of the natural rubber. Thus, the method was not considered to be practically feasible. According to the literature, the tear strength of latex products can be improved by addition of silica gel. However, the use of inorganic additives leads to the generation of ash when rubber products are disposed of by incineration and one great advantage of radiation vulcanization, i.e., a reduced ash content from disposal by incineration, is cancelled.
Under the circumstances, it is an object of the present invention to improve the tear strength of a radiation vulcanized natural rubber latex shaped article that gives rise to a small ash content when disposed of by incineration and which yet is improved in tear strength.